91Ó°ÊÓ

The given diagram is:

From the given diagram it can be noticed that:

$∠AOF=40°$and$∠BOC=60°$

The two angles are said to be congruent angles if they have equal measure.

If two angles A and B are congruent then it can be written as $∠Aâ‰\dots ∠B$.

The theorem 2-3 states that the vertical angles are congruent.

From the diagram, it can be noticed that:

The angles $COD$and $AOF$are vertically opposite angles.

Therefore, from theorem 2-3, it can be said that the angles $COD$and $AOF$are congruent angles.

Therefore, $∠CODâ‰\dots ∠AOF$.

Therefore, $∠COD=∠AOF=40°$.

Therefore, the measure of the angle $COD$is $40°$.

From the given diagram it can be noticed that:

$∠DOB=∠COD+∠BOC$

Therefore,

$∠DOB=∠COD+∠BOC$

$=40+60$

$=100$

Therefore, the measure of the angle $DOB$is $100°$.

The measure of the angle $DOB$is $100°$.

Therefore, the complete statement is: $m∠DOB=100°$.